Person:
Brédas, Jean-Luc

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Author: Heimel, Georg ×

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    First-principles study of the geometric and electronic structure of Au₁₃ clusters: Importance of the prism motif
    (Georgia Institute of Technology, 2008-04) Gruber, Mathis ; Heimel, Georg ; Romaner, Lorenz ; Brédas, Jean-Luc ; Zojer, Egbert
    The geometric structure, symmetry, and spin of Au13 clusters are investigated in the framework of densityfunctional theory, with particular attention paid to the correlation among these properties. Several computational approaches are carefully tested on previously proposed cluster configurations. Complications and possible pitfalls in electronic-structure calculations on these systems are highlighted. Using molecular dynamics with quantum mechanically calculated forces, a set of favored high binding energy geometric structures, where a trigonal prism acts as the central building block, is discussed.
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    A theoretical view on self-assembled monolayers in organic electronic devices
    (Georgia Institute of Technology, 2008-04) Heimel, Georg ; Romaner, Lorenz ; Zojer, Egbert ; Brédas, Jean-Luc
    Self-assembled monolayers (SAMs) of covalently bound organic molecules are rapidly becoming an integral part of organic electronic devices. There, SAMs are employed to tune the work function of the inorganic electrodes in order to adjust the barriers for charge-carrier injection into the active organic layer and thus minimize undesired onset voltages. Moreover, in the context of molecular electronics, the SAM itself can carry device functionality down to a few or even a single molecule. In the present contribution, we review recent theoretical work on SAMs of prototype π-conjugated molecules on noble metals and present new data on additional systems. Based on first-principles calculations, we establish a comprehensive microscopic picture of the interface energetics in these systems, which crucially impact the performance of the specific device configuration the SAM is used in. Particular emphasis is put on the modification of the substrate work function upon SAM formation, the alignment of the molecular levels with the electrode Fermi energy, and the connection between these two quantities. The impact of strong acceptor substitutions is studied with the goal of lowering the energy barrier for the injection of holes from a metallic electrode into the subsequently deposited active layer of an organic electronic device.
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    Impact of bidirectional charge transfer and molecular distortions on the electronic structure of a metal-organic interface
    (Georgia Institute of Technology, 2007-12) Romaner, Lorenz ; Heimel, Georg ; Gerlach, Alexander ; Schreiber, Frank ; Johnson, Robert L. ; Zegenhagen, Joerg ; Duhm, Steffen ; Koch, Norbert ; Zojer, Egbert ; Brédas, Jean-Luc
    Interface energetics are of fundamental importance in organic and molecular electronics. By combining complementary experimental techniques and first-principles calculations, we resolve the complex interplay among several interfacial phenomena that collectively determine the electronic structure of the strong electron acceptor tetrafluoro-tetracyanoquinodimethane chemisorbed on copper. The combination of adsorption-induced geometric distortion of the molecules, metal-to-molecule charge transfer, and molecule-to-metal back transfer leads to a net increase of the metal work function.
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    Interface energetics and level alignment at covalent metal-molecule junctions: pi-conjugated thiols on gold
    (Georgia Institute of Technology, 2006-05) Heimel, Georg ; Romaner, Lorenz ; Zojer, Egbert ; Brédas, Jean-Luc
    The energetics at the interfaces between metal and monolayers of covalently bound organic molecules is studied theoretically. Despite the molecules under consideration displaying very different frontier orbital energies, the highest occupied molecular levels are found to be pinned at a constant energy offset with respect to the metal Fermi level. In contrast, the molecular properties strongly impact the metal work function. These interfacial phenomena are rationalized in terms of charge fluctuations and electrostatics at the atomic length scale as determined by first-principles calculations.
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    Effective conjugation and Raman intensities in oligo(para-phenylene)s: A microscopic view from first-principles calculations
    (Georgia Institute of Technology, 2005-03) Heimel, Georg ; Somitsch, Dieter ; Knoll, Peter ; Brédas, Jean-Luc ; Zojer, Egbert
    Electron-phonon coupling in oligo(para-phenylene)s is addressed in terms of the off-resonance Raman intensities of two distinct modes at 1220 and 1280 cm−1. On the basis of Albrecht’s theory, vibrational coupling and Raman intensities are calculated from first-principles quantum-chemical methods. A few-state model is used to evaluate the dependence of the mode intensities on oligomer length, planarity, and excitation wavelength. The link between electron delocalization∕conjugation and Raman intensities is highlighted. Extending on prior studies, the present work focuses on providing an in-depth understanding of the origin of this correlation in addition to reproducing experimental findings. The model applied here allows us to interpret the results on a microscopic, quantum-mechanical basis and to relate the observed trends to the molecular orbital structure and nature of the excited states in this class of materials. We find quantitative agreement between the results of the calculations and those of measurements performed on oligo(para-phenylene)s of various chain lengths in the solid state and in solution.
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    Breakdown of the mirror image symmetry in the optical absorption/emission spectra of oligo(para-phenylene)s
    (Georgia Institute of Technology, 2005-01) Heimel, Georg ; Daghofer, Maria ; Gierschner, Johannes ; List, Emil J. W. ; Grimsdale, Andrew C. ; Müllen, Klaus ; Beljonne, David ; Brédas, Jean-Luc ; Zojer, Egbert
    The absorption and emission spectra of most luminescent, π-conjugated, organic molecules are the mirror image of each other. In some cases, however, this symmetry is severely broken. In the present work, the asymmetry between the absorption and fluorescence spectra in molecular systems consisting of para-linked phenyl rings is studied. The vibronic structure of the emission and absorption bands is calculated from ab initio quantum chemical methods and a subsequent, rigorous Franck-Condon treatment. Good agreement with experiment is achieved. A clear relation can be established between the strongly anharmonic double-well potential for the phenylene ring librations around the long molecular axis and the observed deviation from the mirror image symmetry. Consequences for related compounds and temperature dependent optical measurements are also discussed